I have a Merida Scultura 100 connected to a Wahoo Kickr. For a few weeks now I've been having problems where, particularly in a certain combination of gears, the chain won't stay put on the rear cogs, but keeps slipping "up".

Specifically, when I am on the larger (higher?) of the two gears in the chainring in the front, and I am switching from the larger to the smaller cogs on the rear (I believe this is 'lower to higher', but I never get it straight -- basically from easier to harder), when I reach the middle set of cogs on the back the chain won't stay there longer than perhaps 15-20 seconds, particularly if I pedal harder. It jumps up to the next smaller cog, then back down, then perhaps even further down, then back up again.

If I keep slight pressure on the gear lever, the one that would make the rear derailleur move the chain down (that is, to a larger cog), but not pressing hard enough to actually make it change gears, then it doesn't jump but instead works just fine. That is, the slight pressure seems to keep it from jumping up.

I'm wondering if there's something on one of the derailleurs that I can adjust, that is, one of the screws? I know there is one that deals with keeping the actual shifting distance correct, so the chain doesn't jump too far, but I don't know if that's what's going on here. (I am wondering whether the chain is loose, but I had it changed a couple of months ago -- I have an appointment at the bike shop in a week or so to have everything checked.)

The cassette on the Wahoo (the ones where the chain is slipping) are only about six months old, so I don't think there's anything wrong with them.

If there's no screw to adjust to solve my problem, then is my method of keeping slight pressure on the shift-lever OK until I get to the bike shop?

EDIT: Just to be clear, everything has worked fine with the bike and the Kickr up until a few weeks ago (that is, for about six months the combo has worked fine). So something's gone a bit out of whack. I'll try the instructions helpfully provided below.

I have a Merida Scultura 100 connected to a Wahoo Kickr. For a few weeks now I've been having problems where, particularly in a certain combination of gears, the chain won't stay put on the rear cogs, but keeps slipping "up".

Specifically, when I am on the larger (higher?) of the two gears in the chainring in the front, and I am switching from the larger to the smaller cogs on the rear (I believe this is 'lower to higher', but I never get it straight -- basically from easier to harder), when I reach the middle set of cogs on the back the chain won't stay there longer than perhaps 15-20 seconds, particularly if I pedal harder. It jumps up to the next smaller cog, then back down, then perhaps even further down, then back up again.

If I keep slight pressure on the gear lever, the one that would make the rear derailleur move the chain down (that is, to a larger cog), but not pressing hard enough to actually make it change gears, then it doesn't jump but instead works just fine. That is, the slight pressure seems to keep it from jumping up.

I'm wondering if there's something on one of the derailleurs that I can adjust, that is, one of the screws? I know there is one that deals with keeping the actual shifting distance correct, so the chain doesn't jump too far, but I don't know if that's what's going on here. (I am wondering whether the chain is loose, but I had it changed a couple of months ago -- I have an appointment at the bike shop in a week or so to have everything checked.)

The cassette on the Wahoo (the ones where the chain is slipping) are only about six months old, so I don't think there's anything wrong with them.

If there's no screw to adjust to solve my problem, then is my method of keeping slight pressure on the shift-lever OK until I get to the bike shop?